When blocking with shield in off hand, left click does nothing. I believe the developers have said multiple times that we would be able to attack while blocking with shields.
Blocking is still a thing! Just not with swords 🙂 You can even block and hit at the same time.
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https://twitter.com/dinnerbone/status/603928005287739392 this is what dinnerbone said back in May. They may have changed their minds, but it was intended at one point to have blocking and hitting occur at the same time.
I feel they just used the sword's blocking code, so it may be works as intended. I think they meant to say that you can have a shield to go along with the sword.